This invention relates to alpha-olefin polymerization processes and particularly relates to catalyst systems in such processes which produce commercially acceptable levels of low molecular weight and, especially, amorphous polymers as determined by the amount of polymerized product which is soluble in n-hexane.
The polymerization of alpha-olefins to normally-solid, substantially crystalline polymers using heterogeneous catalysts comprising transition metal halides and aluminum alkyls now is well-known in the art. However, there is a continuing need in the industry for catalyst systems which have a high yield, as measured by the grams of crystalline product per gram of transition metal halide consumed, while producing a minimum amount of n-hexane-soluble polymer. In a slurry polymerization process which uses a hydrocarbon solvent, such as n-hexane, amorphous and low molecular weight polymer accumulates in the solvent which necessitates extensive solvent purification procedures. Since the economic value of such hexane-soluble polymer is lower than normally-solid, substantially crystalline product, the overall process becomes less efficient as the amount of n-hexane-soluble product increases.
In a solventless, liquid-phase bulk polymerization or in a vapor phase process, the production of polymers which contain more than about 2% of n-hexane-soluble product requires a separate extraction procedure to produce commercially acceptable products and makes these inherently efficient processes uneconomical. Therefore, polymerization processes which produce low amounts of n-hexane-soluble polymer while not adversely affecting polymerization yield are in demand.
Various catalyst components in addition to transition metal halides and aluminum alkyls have been disclosed to minimize n-hexane-soluble products in alpha-olefin polymerization. Such additional components include aliphatic amines, tetraorganosilyl compounds such as tetraalkyl-, tetraaryl- and tetraalkoxysilanes, sterically hindered cyclic amines, amine N-oxides and organotin sulfides. One such system is described in U.S. Pat. No. 3,950,268 incorporated herein by reference. 2-Methyl-tetrahydrofuran is disclosed as an additive in propylene polymerization in U.S. Pat. No. 3,441,551.
It is an object of this invention to discover catalyst additives which decrease n-hexane-soluble products while maintaining reasonable polymerization activity.